Both are commonly used. 你来自哪里，别人可以回答来自某个国家、城市，某个省或者是某个地区。而你是哪国人，别人只能回答国家。

1. if you want to ask a foreigner's nationality, use "你是哪个国家的？" 2. if you want to ask a chinese which part of China he/she comes from, use "你是哪里人？" 3. if you don't know he is a foreigner or chinese, first use "你是哪里人？", if he told you he is a foreigner, then use "你是哪个国家的？" but in China is very easy to distinguish chinese and foreigner, except Korean and Japanese people.

你是哪国人比较好。你来自哪里不会这么说的，这是纯外语的译法。如果这么问会说你是哪里人或是你是哪儿来的

其实也经常用‘你来自哪里？“呢~^_^

“你来自那里”，会比较口语化一点，“你是哪国人”，一般是问外国朋友的。